Steel surfaces, such as those of the cladding tubes or internal structures in reactors that are operated with molten lead or lead alloys, are subject to corrosive attack by the molten metal. Gradual destruction of the metal components takes place by dissolution of alloy elements of the steel. This process can be prevented if oxygen is dissolved in the lead or its alloys, specifically at a concentration at which oxide layers form on the surface and suppress the dissolution process. The oxygen concentration, however, preferably does not exceed the value at which oxides of the lead alloys form.
In the presence of lead alloys that contain oxygen at a concentration at which the steel oxidizes but the lead alloys do not, the diffusion of iron ions at the surface produces magnetite layers, and the diffusion of oxygen into the metal produces spinel layers beneath the surface that continually grow over time. This process prevents dissolution attack by the lead alloys, but after an extended time results in a considerable degradation of material properties due to oxidation. In addition, a reliable protective effect is thereby achieved only for temperatures below 500° C. Because higher temperatures may be expected on cladding tubes in a reactor, additional protective layers must be considered for higher temperatures. The use of aluminum-containing alloys that, by the selective formation of thin aluminum oxide layers, represent an outstanding diffusion barrier again dissolution attack by the molten metal and against progressive oxidation of the steel, is very promising in this regard. It is not practical to add aluminum as an alloy to the entire steel matrix, since it is known that aluminum causes severe embrittlement of steel. The aluminum component must therefore be limited to a thin superficial layer. Two methods have been tested for the production of aluminum-containing surface alloys: industrial “pack cementation” and, on a laboratory scale, surface melt alloying using pulsed electron beams (GESA method). “Pack cementation” is a high-temperature diffusion method in which aluminum is diffused at approximately 900° C., in the gas phase, into the steel matrix. In the GESA method, a previously applied aluminum layer or thin film is alloyed into the steel surface with the aid of a pulsed electron beam. The basic protective mechanism has been confirmed on specimens treated with both methods. Transfer of the two methods for use on real thin-walled tubes has, however, been unsatisfactory. Because of the high process temperature for the diffusion method, utilization may be limited to nickel-containing austenitic steels. The dimensional stability of the cladding tubes cannot be guaranteed because of the high temperature. The surface alloy layer furthermore contains nickel. Of all the alloy constituents of steel, nickel has the highest solubility in lead, so that long-term durability in lead and lead alloys becomes questionable. On the other hand, the GESA method has the disadvantage that, because of the limited physical extent of the electron beam, the elongated cladding tube must be treated in overlapping fashion with multiple pulses. A portion of the previously applied aluminum layer can vaporize in the overlap areas, so that locally, insufficient aluminum is alloyed into the steel surface. It is desirable to prevent such local defects.